908  
FXUS02 KWBC 120654  
PMDEPD  
 
EXTENDED FORECAST DISCUSSION  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
253 AM EDT SUN AUG 12 2018  
 
VALID 12Z WED AUG 15 2018 - 12Z SUN AUG 19 2018  
   
..PATTERN OVERVIEW AND GUIDANCE/PREDICTABILITY ASSESSMENT
 
 
WITH THE PRIMARY CORRIDOR OF THE WESTERLIES JUXTAPOSED NORTH OF  
THE INTERNATIONAL BORDER WITH CANADA, A SERIES OF SLOW MOVING  
FEATURES ARE PRIMED TO DRIFT EASTWARD ACROSS THE NATION NEXT WEEK.  
PINPOINTING WHICH SHORTWAVE WILL DOMINATE IS DIFFICULT TO SAY  
GIVEN A LACK OF RUN TO RUN MODEL CONTINUITY IN CONJUNCTION WITH  
THE WEAK STEERING FLOWS TO CONTEST WITH. WHILE THE INITIAL IMPULSE  
IN THE CHAIN SHOULD BE EXITING OFF THE MID-ATLANTIC COAST ON  
WEDNESDAY, THE FOCUS SHIFTS TOWARD THE NEXT ELONGATED TROUGH  
PUSHING ACROSS THE NORTH-CENTRAL U.S. THE GUIDANCE CONTINUES TO  
STRUGGLE WITH HOW AMPLIFIED THE FLOW WILL BECOME AS IT MOVES  
DOWNSTREAM INTO THE OHIO RIVER VALLEY LATE IN THE WORK WEEK.  
ENSEMBLE SPAGHETTI PLOTS SHOW A MIXED BAG OF SCENARIOS ALTHOUGH A  
VAST MAJORITY OF THESE SUPPORT SOME SURFACE WAVE TRACKING TOWARD  
THE LOWER GREAT LAKES ON THURSDAY NIGHT INTO FRIDAY MORNING. ITS  
SLOW FORWARD MOTION SHOULD BRING IT ACROSS THE VICINITY OF NEW  
ENGLAND BY DAY 7/AUGUST 19. DIFFICULT TO SAY HOW DEEP ITS PARENT  
CYCLONE WILL BE GIVEN THE FLUCTUATIONS IN THE GUIDANCE. EVENTUALLY  
RENEWED TROUGHING COULD EMERGE NEXT WEEKEND ACROSS THE CENTRAL  
U.S. ALTHOUGH THIS REMAINS FAR FROM CERTAIN. ENSEMBLE SPAGHETTI  
PLOTS DEPICTS GENERAL WEST-NORTHWESTERLY ALOFT WITH PERHAPS A  
SHORTWAVE EMBEDDED WITHIN IT. FINALLY ACROSS WESTERN NORTH  
AMERICA, MULTI-DAY ENSEMBLE COMPARISONS HAVE SHOWN THE ECMWF MEANS  
TO REALLY BACK OFF ON THE POLEWARD EXTENT OF THE BUILDING RIDGE.  
THE CORE OF THE UPPER RIDGE SHOULD STAY CONFINED TO THE  
SOUTHWESTERN U.S. WHILE A RIPPLE IN THE FLOW COULD PUSH INTO THE  
PACIFIC NORTHWEST NEXT WEEKEND. HOWEVER, THIS REMAINS THE MOST  
VOLATILE SECTOR OF THE MAP SO A LOT CAN CHANGE IN THE UPCOMING  
DAYS.  
 
THE PREFERRED SOLUTION UTILIZED A MULTI-OPERATIONAL MODEL BLEND  
THROUGH DAY 4/AUGUST 16 LED BY THE 18Z/12Z GFS AND 12Z ECMWF WITH  
SMALLER CONTRIBUTIONS FROM THE 12Z UKMET/CMC. THEREAFTER, BUILDING  
SPREAD ACROSS WESTERN/CENTRAL NORTH AMERICA SUPPORTED A MORE  
ENSEMBLE BASED FORECAST STRATEGY. GENERALLY COMPROMISED BETWEEN  
THE OUTPUT FROM THE 18Z GEFS/12Z NAEFS MEANS WITH THE 12Z ECMWF  
ENSEMBLE MEAN. CONFIDENCE REMAINS NEAR AVERAGE ALTHOUGH ENSEMBLE  
SPAGHETTI PLOTS SHOW QUITE A BIT OF NOISE IN THE PATTERN NEXT  
WEEKEND WHICH SUGGESTS SLIGHTLY REDUCED CERTAINTY.  
 
   
..WEATHER HIGHLIGHTS AND HAZARDS
 
 
GIVEN THE PERSISTENCE OF SHORTWAVES EMERGING OUT OF THE CENTRAL  
U.S., DAILY RAINFALL THREATS SHOULD MAINTAIN SLIGHTLY BELOW  
AVERAGE TEMPERATURES FROM THE MIDDLE OF THE COUNTRY EASTWARD  
TOWARD THE OHIO/TENNESSEE RIVER VALLEYS. SUCH DEPARTURES SHOULD  
NOT BE TREMENDOUS BY ANY MEANS WITH WIDESPREAD NUMBERS UNLIKELY TO  
EXCEED 5 DEGREES. OTHERWISE, THE HEAT IS EXPECTED TO REMAIN  
CONFINED TO AREAS OF THE WESTERN STATES. ABOVE AVERAGE HEIGHTS  
WILL SUPPORT TOASTY READINGS, GENERALLY AROUND 5 TO 10 DEGREES  
ABOVE CLIMATOLOGY, PARTICULARLY OVER THE PACIFIC NORTHWEST.  
THROUGH FRIDAY, THIS WILL ALSO EXTEND EASTWARD INTO THE NORTHERN  
ROCKIES AS WELL AS THE NORTH-CENTRAL STATES.  
 
REGARDING PRECIPITATION THREATS, DAILY BOUTS OF MODERATE TO  
LOCALLY HEAVY RAINFALL ARE LIKELY FROM OKLAHOMA/KANSAS THROUGH  
MISSOURI AND INTO THE OHIO VALLEY/LOWER GREAT LAKES REGION.  
SURROUNDING LOCATIONS TO THIS HEAVIER SWATH CAN EXPECT THE USUAL  
DIURNALLY FORCED CONVECTION ALTHOUGH AREAL AVERAGE AMOUNTS SHOULD  
BE A BIT LIGHTER IN NATURE. ACROSS THE FOUR CORNERS REGION, ACTIVE  
MONSOONAL FLOW WILL DRIVE PERIODS OF SHOWERS AND THUNDERSTORMS  
WHICH SHOULD SUBSIDE LATE INTO THE NIGHT.  
 
RUBIN-OSTER  
 
WPC MEDIUM RANGE 500 MB HEIGHTS, SURFACE SYSTEMS, WEATHER GRIDS,  
QUANTITATIVE PRECIPITATION, WINTER WEATHER OUTLOOK PROBABILITIES  
AND HEAT INDEXES ARE FOUND AT:  
 
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST500_WBG.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/MEDR/5DAYFCST_WBG_CONUS.GIF  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/5KM_GRIDS/5KM_GRIDSBODY.HTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/QPF/DAY4-7.SHTML  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/WWD/PWPF_D47/PWPF_MEDR.PHP?DAY=4  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/HEAT_INDEX.SHTML  
 

 
 
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